1. Field of the Invention
The present invention relates to voltage regulator circuits, and more specifically, to a voltage regulator circuit for RF signals.
2. Description of Related Art
Radio frequency identification system is an automatic identification method that involves affixing a small electronic tag to a product which may be checked and monitored by a device known as “reader” which in turn transmits the data stored in the electronic tag back to the system via a wireless RF means, thus achieving remote authentication, tracking, control, management and handling.
The electronic tags can be categorized into two general varieties, passive or active. In particular, passive RFID tags have no internal power supply. The minute electrical current induced in the antenna by the incoming radio-frequency (RF) signal provides just enough power for the CMOS integrated circuit in the tag to power up and transmit a response.
Upon receiving the RF signal by the antenna of passive RFID tags, a rectifier is used to convert the RF signal to a DC voltage level first and subsequently the capacitors are charged repeatedly to increase the DC voltage to a sufficient level for the next stage of circuit operation.
However, after the rectifier converts the RF signal to a DC voltage level, the voltage level is often perturbed by ripple effect or fluctuates easily due to the effect of external temperature. Hence, a passive RFID tag usually requires a voltage regulator circuit design to mitigate the effects of ripple perturbation or external temperature.
Referring to FIG. 1, a schematic of the first conventional voltage regulator circuit is illustrated. As illustrated in the diagram, the front end of voltage regulator circuit has four diodes to avoid damages to the backend due to excessive input power. The uniqueness of the reference voltage circuit is its multi-stage current mirror cascode and that the NMOS operates in the sub-threshold region, thereby decreasing the operating power consumption.
However, the aforementioned circuit structure does not take into account of the effect that the environment temperature has on the voltage regulator circuit; therefore, such a circuit structure does not include components that generate negative temperature coefficient and is unable to mitigate the effect that the external temperature has on the voltage regulator circuit, thereby limiting the applications of such a voltage regulator circuit.
Referring to FIG. 2, a schematic of the second conventional voltage regulator circuit is shown. As illustrated in the diagram, a zero is generated by the internal circuit of the voltage regulator circuit for the purpose of frequency compensation, thereby replacing the conventional way of using the equivalent series resistor of the backend circuit for providing zero compensation. Hence the transient response of the voltage regulator as well as the noise interference at the backend of the circuit are significantly reduced.
However, the aforesaid circuit structure does not take into account of the effect that ripples of the original RF signals have on the voltage regulator circuit. In addition, the error amplifier design does not adopt the cascode connection, therefore during operation, the perturbation of the voltage regulator circuit caused by ripple voltages cannot be avoided.
In summary, it has become an urgent issue to designers of the RF circuit design field to propose a circuit with voltage regulation function that generates a reference current free from temperature effect and ripple voltage perturbation, thereby providing a precise error voltage calibration and shortening the calibration time for calibrating the voltage regulator circuit errors. Furthermore, regulation of a voltage level free of temperature effect as well as the ripple voltage perturbation is achieved.